Conductive sheet material

ABSTRACT

A conductive microporous sheet material comprises primary carbon fibres having a cross-sectional dimension of at least 1 μm, secondary carbon fibres in the form of carbon nanofibres and a binding agent for binding said primary and secondary fibres. The material may be produced by a wet-laid non-woven (paper-making) process. The sheet material may be used as a gas diffusion layer for a fuel cell or an electrode material for a battery.

[0001] The present invention relates to a conductive microporous sheetmaterial for use in electrical devices, particularly but not exclusivelyfor batteries and related devices.

[0002] There is an increasing demand for sheet materials which have amicroporous structure combined with electrical conductivity and a highlevel of chemical resistance. Such materials find application as gasdiffusion layers for fuel cells and as electrode materials forbatteries. The present invention seeks to provide materials which meetthese requirements as well as a method for the manufacture of suchmaterials.

[0003] According to the present invention there is provided a conductivemicroporous sheet material comprising primary carbon fibres having across-sectional dimension of at least 1 μm, secondary carbon fibres inthe form of carbon nanofibres and a binding agent for binding saidprimary and secondary fibres.

[0004] The sheet of the invention has a microporous structure determinedprimarily by the relative proportions of the first and second fibres.The variation in pore structure with the secondary (nanofibre) contentmay readily be determined experimentally by a person skilled in the art.Thus, for example, the sheet may be produced by a wet-laying technique(see below) and the experimental determination may be effected byproducing and testing laboratory produced single sheets (hard sheets).This information may then be used to select the appropriate blend ofprimary and secondary fibres for a given microporous structure.

[0005] The sheet of the invention may be a thin, flexible material.

[0006] The primary fibres preferably have a cross-section of 1 to 15 μm,more preferably 4 to 12 μm, even more preferably 5 to 10 μm. Typicallythe primary fibres will have a length of a few millimetres, e.g. 3 to 8mm (about 6 mm). A preferred example of primary carbon fibre is SGL C25(available from Technical Fibre Products Ltd.).

[0007] The primary fibres may be obtained from acrylonitrile or pitch.

[0008] The secondary fibres (nanofibres) preferably have a cross sectionof between 100 and 500 nanometres, more preferably between 100 and 250nanometres. The nanofibres may be produced by vapour deposition. Apreferred example of a carbon nanofibre is Pyrograf-III (available fromASI).

[0009] Preferably the primary carbon fibre constitutes between 10 and 90wt % of the total weight of fibres and secondary fibres constitutebetween 10 and 90 wt % on the same basis. Preferably the fibres togetherprovide at least 90% by weight of the sheet material.

[0010] The binding agent is required for adequate bonding strength ofthe material. The binding agent will generally constitute less than 10%by weight of the sheet material, and more typically less than 5% on thesame basis.

[0011] The binding agent may for example be a thermoplastic orthermosetting resin, a suitable example of which is a phenolic resinsuch as GP5520. Whilst the use of resin binding agent is perfectlysatisfactory, improved conductivity can generally be achieved by use ofcarbon as the binding agent. Sheets utilising carbon as the bindingagent may be produced by heat treatment in an inert atmosphere of asheet material incorporating a resin binding agent, said conversion ofthe resin binding agent to carbon serving to increase conductivitywhilst retaining the controlled microporous structure.

[0012] A sheet according to the present invention may have any one orany combination of the following properties:—

[0013] A. A weight of between 10 and 200 g/m², more preferably about 50g/m².

[0014] B. A thickness of between 0:1 and 2 mm, more preferably about 0.3mm.

[0015] C. A Gurley air permeability of between 8 and 50 seconds/300 cm³.

[0016] D. A maximum pore size of less than 22 μm, more preferably lessthan 16 μm and most preferably less than 12 μm.

[0017] E. A through plane resistance of less than 150 Ω/cm, morepreferably less than 50 Ω/cm.

[0018] F. A tensile strength of between 0.7 and 1.3 kN/m.

[0019] Conductive sheet material in accordance with the invention has avariety of end uses, including:—

[0020] (1) Gas diffusion layers for fuel cells

[0021] (2) Electrode materials for batteries.

[0022] The conductive sheet material, according to the invention ispreferably produced using a wet-laid non-woven (papermaking) process.The use of a wet-laid production process allows a wide range ofproportions of carbon fibres and carbon nanofibres to be used and thuslends itself to production of materials with highly specific porestructures.

[0023] The preferred method of manufacture is to form a slurry of thetwo fibre types with binder by mixing the materials in water at aconcentration of up to 1% by weight (e.g. between 0.02 and 0.5 wt %).Mixing is preferably carried out using a high speed agitator and theresulting slurry is formed into a suitable sheet material by passingthrough a papermaking former.

[0024] Fibre distribution and sheet forming may be aided by the use ofviscosity modifiers and/or drainage aids.

[0025] After forming liquid may be removed from the sheet by vacuumand/or hot air drying. Where both liquid removal methods are used it ispreferred that hot air drying is applied ultimately as it may be used tomelt or cure the binder. It is preferred that the final stage of theproduction process is the carbonisation of the binding agent.

[0026] Following the carbonisation stage the sheet material ispreferably formed into a continuous roll in order to facilitate furtherautomated processing.

[0027] Both continuous of batch processing of the sheet material areenvisaged.

[0028] Preferably in the production process the binder is initially theform of a powder although the use of a binder in any other physical formis not precluded.

[0029] The invention will now be described further with reference to thefollowing non-limiting Examples.

EXAMPLE 1

[0030] A sheet was formed by mixing the following elements in waterusing a high speed agitator at a combined concentration of 0.5 wt %.Carbon fibre (SGL C25), 6 mm chopped length 24 wt % Carbon nanofibre(Pyrograf-III, ex ASI) 73 wt % Phenolic resin (GP 5520)  2 wt %

[0031] The resulting material was converted into sheet form using apapermaking former. The sheet was dried using a combination of vacuumand hot air and then carbonised by heating in an inert atmosphere untilthe phenolic binder was completely converted to carbon.

[0032] Sheets formed from the above mixture had the followingcharacteristics:— Weight 50 g/m² Thickness 0.3 mm Tensile strength 0.7kN/m Gurley air permeability 50 seconds/300 cm³ Maximum pore size 12 μmThrough plane resistance 150 Ω/cm

EXAMPLE 2

[0033] A sheet was formed by mixing the following elements using thesame technique as in Example 1. Carbon fibre (SGL C25), 6mm choppedlength 49 wt % Carbon nanofibre (Pyrograf-III, ex ASI) 49 wt % Phenolicresin (GP 5520)  2 wt %

[0034] Sheets formed from the above mixture had the followingcharacteristics:— Weight 50 g/m² Thickness 0.3 mm Tensile strength 1.0kN/m Gurley air permeability 20 seconds / 300 cm³ Maximum pore size 16μm Through plane resistance 150 Ω/cm

EXAMPLE 3

[0035] A sheet was formed by mixing the following elements using thesame technique as in Example 1. Carbon fibre (SGL C25), 6mm choppedlength 74 wt % Carbon nanofibre (Pyrograf-III, ex ASI) 24 wt % Phenolicresin (GP 5520)  2 wt %

[0036] Sheets formed from the above mixture had the followingcharacteristics:— Weight 50 g/m² Thickness 0.3 mm Tensile strength 1.3kN/m Gurley air permeability 8 seconds / 300 cm³ Maximum pore size 22 μmThrough plane resistance 150 Ω/cm

1) a conductive microporous sheet material comprising primary carbonfibres having a cross-sectional dimension of at least 1 μm, secondarycarbon fibres in the form of carbon nanofibres and a binding agent forbinding said primary and secondary fibres. 2) A sheet material accordingto claim 1, wherein the primary carbon fibres have a cross section ofbetween 4 and 12 μm. 3) A sheet material according to claim 2, whereinthe primary carbon fibres have a cross section of between 5 and 10 μm.4) A sheet material according to claim 1, 2 or 3, wherein the primarycarbon fibres are between 3 and 8 mm in length. 5) A sheet materialaccording to claim 4, wherein the primary carbon fibres are about 6 mmin length. 6) A sheet material according to any one of claims 1 to 5,wherein the secondary carbon fibres have a cross section of between 100and 500 nanometres. 7) A sheet material according to claim 6, whereinthe secondary carbon fibres have a cross section of between 100 and 250nanometres. 8) A sheet material according to any one of claims 1 to 7,wherein the primary carbon fibres constitute between 10 and 90 wt % ofthe total weight of fibres. 9) A sheet material according to any one ofclaims 1 to 8, wherein the secondary carbon fibres constitute between 10and 90 wt % of the total weight of fibres. 10) A sheet materialaccording to any one of claims 1 to 9, wherein the binding agentconstitutes less than 10 wt % of the sheet material. 11) A sheetmaterial according to claim 10, wherein the binding agent constitutesless than 5 wt % of the sheet material. 12) A sheet material accordingto any one of claims 1 to 11, wherein the binding agent is selected fromthe group comprising thermoplastic resins and thermosetting resins. 13)A sheet material according to claim 12, wherein the binding agent is aphenolic binder. 14) A sheet material according to any of the precedingclaims, wherein the binding agent is carbon. 15) A sheet materialaccording to any one of claims 1 to 14, wherein the sheet has a weightof between 10 and 200 g/m². 16) A sheet material according to claim 15,wherein the sheet has a weight of about 50 g/m². 17) A sheet materialaccording to any one of claims 1 to 16, wherein the sheet has athickness of between 0.1 and 2 mm. 18) A sheet material according toclaim 17, wherein the sheet has a thickness of about 0.3 mm. 19) A sheetmaterial according to any one of claims 1 to 18, wherein the sheet has aGurley air permeability of between 8 and 50 seconds/300 cm³. 20) A sheetmaterial according to any one of claims 1 to 19, wherein the sheet has amaximum pore size of less than 22 μm. 21) A sheet material according toclaim 20, wherein the sheet has a maximum pore size of less than 16 μm.22) A sheet material according to claim 21, wherein the sheet has amaximum pore size of less than 12 μm. 23) A sheet material according toany one of claims 1 to 22, wherein the sheet has a through planeresistance of less than 150 Ω/cm. 24) A sheet material according toclaim 23, wherein the sheet has a through plane resistance of less than50 Ω/cm. 25) A sheet material according to any one of claims 1 to 24,wherein the sheet has a tensile strength of between 0.7 and 1.3 kN/m.26) A battery which includes a portion of sheet material according toany one of claims 1 to
 25. 27) A method of producing a porous conductivesheet according to claim 1 comprising:— (a) forming an aqueous slurry ofthe primary and secondary carbon fibres with a binding agent; (b)applying the slurry to a paper forming screen to produce a sheetthereof; and (c) drying the sheet. 28) A method according to claim 27,wherein in step (a) the combined weight of the primary and secondaryfibres and the binding agent constitutes between 0.02 and 0.5 wt % ofthe aqueous slurry. 29) A method according to claim 27 or 28, whereinthe drying step is carried out by hot air heating or vacuum technique.30) A method according to claim 27, 28 or 29, which also includes anadditional step (d) of carbonising the sheet material. 31) A methodaccording to claim 30 wherein the carbonisation is carried out at anelevated temperature in an inert atmosphere. 32) A method according toany one of claims 27 to 31, wherein the fibre distribution and sheetforming is aided by the use of viscosity modifiers, drainage aids or acombination of both.